1. Field of the Invention
The present invention relates generally to aircraft control surfaces and, more particularly, to a seamless control surface for an airfoil so constructed that the outer surface curvature of the airfoil and of the seamless control surface is smooth and continuous over substantially the entirety thereof at all positions of the seamless control surface.
2. Description of the Prior Art
Conventional control surfaces are attached to the wing by hinges and typically operated by hydraulic or pneumatic actuators. When flaps are deployed, there exists a discontinuity in slope resulting in flow separation. The following disadvantages are observable with respect to conventional flaps:
hinged control surfaces drastically reduce aerodynamic efficiency of the control surfaces due to flow separation along the hinge line; PA1 edge or tip vortices reduce lift; PA1 hydraulic actuators are heavy and their frequency response characteristics are low. Because of this, hydraulic actuated control surfaces encounter `buzz` in transonic flight regimes and have a reduced flutter speed; PA1 control surface stiffness must be increased to increase aeroelastic stability characteristics; this increases net weight of the control actuator system (including weight of the hydraulic or pneumatic actuator) as well as the net weight of the vehicle; PA1 agility of the aircraft is limited due to reduced aerodynamic effectiveness of the hinged control surface; and PA1 limited applicability of the active flutter suppression algorithm due to the slow response of hydraulic and pneumatic systems.
A number of patents disclosing typical inventions pertinent to the present invention will now be presented. For instance, in U.S. Pat. No. 5,531,407 to Austin et al., a plurality of translational actuators capable of extending and contracting are used to control the shape of a structure having one or more surfaces by means of a plurality of internal actuators. The shapes of the surfaces are controlled by computing the actuator strokes or loads required for achieving specified surface deflections. Two disclosed methods accomplish this control. In the actuator stroke-control method the surface deflections or deflection errors for closed-loop control are multiplied by a stroke-control gain matrix which is a function of the properties of the structure with the actuators absent. In the actuator load-control method, the surface deflections or deflection errors for closed-loop control are multiplied by a load-control gain matrix which is a function of the properties of the structure with the actuators absent. The control gain matrices minimize the surface shape errors. Ratios of stresses to allowable values are continuously monitored throughout the structure and corrective action is taken to prevent an overstressed condition. Unfortunately, in actuality, a desired airfoil shape cannot be derived by pushing and pulling the surfaces. Moreover, an excessive amount of power would be required to change the shape of a wing structure which is designed to carry heavy loads. In addition, there is no point in changing the wing shape in the center section since it would be aerodynamically undesirable. Leading and trailing edges are the optimum regions for making adjustments; they are the easiest to shape in order to achieve aerodynamic advantages and are also the most cost effective.
U.S. Pat. No. 5,374,011 to Lazanis et al. discloses an adaptive sheet structure with distributed strain actuators which is controlled by a dynamic compensator that implements multiple input, multiple output control laws derived by model-based control methodologies. An adaptive lifting surface is controlled for maneuver enhancement, flutter and vibration suppression and gust and load alleviation with piezoceramic elements located within, or enclosed by sheets of composite material at a particular height above the structure's neutral axis. Sensors detect the amplitudes of lower order structural modes, and distributed actuators drive or damp these and other modes. The controller is constricted from an experimental and theoretical model using conventional control software, with a number of event recognition patterns and control algorithms programmed for regulating the surface to avoid instabilities. The number of control states of the compensator is then reduced by removing states having negligible effects on the plant, and a smaller set of control laws are optimized and then adjusted based on analytical models bench and wind-tunnel testing.
While the device of this patent may work well on test models, it is not practical for aircraft configurations. First, there is weight penalty, and secondly piezoceramic strains are very small such that the desired deformation cannot be achieved. Indeed, any actuator system which employs piezoelectric or terfenol concepts must be built with mechanical magnification factors in the order of 100 or more. There is no indication in the patent that this has been considered.
In U.S. Pat. No. 5,222,699 to Albach et al., the inventive concept is based on use of a warped airfoil shape to eliminate the gaps and abrupt changes that occur at the hinged area of a conventional control surface, and an elastomeric transition session to provide a smooth transition between the warped and undetected shapes of the fixed wing or tail surface sections. The warped shape of the control surface is achieved by mechanically shortening or lengthening either one of the surfaces (upper or lower) of the control surface while the other surface defects to the warp shape. Air loads are supported by the warped surface skins which act as a truss. In addition, the upper and lower skins are connected by elastomeric spars which provide a tension connection but very low shear connection between the skins. An aft facing seal is provided on the surface that is shortened or lengthened to prevent airflow into the airfoil interior area.
Although it is a thought provoking idea, its application to real aircraft would necessarily involve extensive manufacturing considerations and require a costly drive mechanism to achieve.
U.S. Pat. No. 5,150,864 to Roglin et al. discloses an airfoil camber control apparatus which utilizes a cable of shape memory alloy affixed at its ends to a front interior portion of the airfoil. A tensioning system is connected to a rear interior portion of the airfoil and to the cable. When electrical current is applied to the cable to heat it, it returns to its remembered, shorter length, thereby applying tension to the tensioning system to alter the position of the rear portion of the airfoil relative to the front portion.
While this concept may work well for slow shape changes, in that the rate of operation is limited by the heating and cooling cycle, it is inadequate, for flight maneuvers of high performance aircraft, failing to take into account aeroelastic considerations.
U.S. Pat. No. 5,082,207 to Tulinius discloses a system for controlling an aircraft by aeroelastic deflections of the wings which is effective beyond control surface reversal. The system includes flexible wings, leading and trailing edge control surfaces attached to the wings, sensors to measure selected aircraft flight parameters, an information processing system to receive and process pilot command signals and signals from the sensors, and control mechanisms in the wing that respond to processed signals from the information processing system. The control mechanisms selectively position the control surfaces to produce loads such that the wings are deflected in a desired manner for aircraft control. The system can be used for aircraft control (including maintaining stability), optimum cruise, and maneuver performance. Augmentation can be added for maneuver load control, gust load alleviation, and flutter suppression.
This invention utilizes modern active flight control technology in which a number of smaller conventional control surfaces are activated to redistribute the air load so that desired roll rate and flutter suppression can be achieved within the domain of flight envelope. However, the specific mechanisms utilized are not described.
U.S. Pat. No. 4,667,898 to Greenhalgh discloses a remotely piloted vehicle provided with single surface membranous airfoils controllable in flight. The airfoils are selectively deployed from a stowed position on either side within the fuselage by spars attached to the leading edges. Pivotal members attached to the root edges of the airfoils are positioned to regulate twist distribution, angle of attack, root camber ratio and root camber distribution.
This invention provides a deformable camber surface such as those used in handgliders and remotely piloted models and micro airplanes but is not reasonably applicable to conventional aircraft.
U.S. Pat. No. 4,247,066 to Frost et al. discloses an airfoil device and method providing smooth, continuous, variation in airfoil camber and surface curvature over substantially the entire length of the device by use of a trusslike bendable beam as an airfoil rib having the airfoil skin surfaces flexibly slidable relative thereto. The beam is divided chordwise into upper and lower beam members each formed of a plurality of articulated sections.
The beam members are connected by a bendable jackscrew which upon rotation causes one member to move chordwise, and its curvature to be changed, relative to the other thereby effecting deflection of the airfoil with concomitant variation in its camber and the curvatures of its outer skin surfaces.
This device appears to be heavy and cumbersome and requires a thicker airfoil to accommodate it. Moreover, because of the weight increase in the control surface area, it is susceptible to control surface flutter. Furthermore, the actuation rate of any mechanical device is slow hence these devices are not applicable to aeroservoelastic control which enhances the flutter speed margin.
U.S. Pat. No. 3,930,626 to Croswell, Jr. discloses a control surface provided with structural wires generally extending from the leading edge to the trailing edge on both the upper and lower surfaces, or closely adjacent thereto, so that upon selectively heating either the upper or lower wires, they will expand to a greater extent than the unheated wires on the opposite surface to warp or curve the surface generally over its entire extent from its leading edge to its trailing edge. With this structure, it is seen that the upper and lower surfaces will at all times be smooth and there will be no abrupt transition portions, and the general thickness of the wing may be maintained.
Although the concept is sound theoretically, it lacks practicality and has not been put into practice over the many years since it first appeared.
Performance characteristics of any aircraft are based on the quality and distribution of air flow on the lifting surfaces. By nature, birds are able to configure their wings in such a manner that the air flow quality is good and their flying efficiency is optimized. To simulate birds like flying characteristics the lifting surfaces must be able to deform smoothly at appropriate locations. In mid 1980s, Air Force sponsored a mission adaptive wing (MAW) project to study aerodynamic and maneuver performance characteristics of tactical aircraft. An F-111 aircraft was selected and fitted with hydraulic actuators to deform the wing. This aircraft was test flown for various mission performance evaluations. The test results showed overwhelming aerodynamic performance benefits and agility characteristics. However, the actuation system was heavy and expensive to operate, hence, practical implementation of this concept could not be realized at that time.
Recent wind tunnel studies sponsored by ARPA and Air Force show how smoothly contoured control surfaces promote incremental growth in suction pressure near the leading edge. This has beneficial effect on control surface effectiveness leading to enhanced aircraft maneuver performance.
Today with the advent of new materials technology, it is possible to design smoothly deforming lifting surfaces. Solid state actuators, in particular, are being developed which can output large forces at any desired rates. These can also be built in relatively small sizes and light weight. Since, these actuators are small and light, a large number of these actuators can be used on the lifting surface so that any desired lifting surface deformation shape can be commanded for any given flight mission maneuver.
It was with knowledge of the foregoing state of the technology that the present invention has been conceived and is now reduced to practice. The actuation and deployment concept embodied by this invention is different from all of the devices reviewed above. Furthermore, this system can easily be implemented in existing as well as in new aircraft without significant alterations in the design.